Recently, diverse wireless communication technologies are under development in line with the advancement of information communication technology. Among them, a wireless local area network (WLAN) is a technique allowing mobile terminals such as personal digital assistants (PDAs), lap top computers, portable multimedia players (PMPs), and the like, to wirelessly access the Internet at homes, in offices, or in a particular service providing area, based on a radio frequency technology.
Since IEEE (Institute of Electrical and Electronics Engineers) 802, a standardization organization of a WLAN technique, was established in February 1980, a great deal of standardization works have been conducted. The early WLAN technique supported the rate of 1˜2 Mbps through frequency hopping, spread spectrum, infrared communications, and the like, by using a 2.4 GHz frequency based on IEEE 802.11, and recently, a maximum rate of 54 Mbps can be supported by employing orthogonal frequency division multiplex (OFDM) technology to the WLAN. Further, IEEE 802.11 are putting standards of various techniques, such as improvement of quality of service (QoS), allowing for compatibility of access point (AP) protocols, achievement of security enhancement, measurement radio resource measurement, wireless access vehicular environment, ensuring fast roaming, establishing a mesh network, interworking with an external network, wireless network management, and the like, into practical use or are still developing them.
Among the IEEE 802.11, IEEE 802.11b supports a maximum of 11 Mbs communication speed by using the frequency band of 2.4 GHz. IEEE 802.11a, which has been commercialized following the IEEE 802.11b, uses the frequency band of 5 GHz, rather than 2.4 GHz, to reduce the influence of interference compared with the considerably congested frequency band of 2.4 GHz, and has a communication speed increased up to a maximum 54 Mbps by using the OFDM technique. However, IEEE 802.11a has shortcomings in that its communication distance is shorter than that of IEEE 802.11b. Meanwhile, IEEE 802.1g uses the frequency band of 2.4 GHz, like IEEE 802.11b does, to implement a communication speed of a maximum 54 Mbps and satisfies backward compatibility, and as such, IEEE 802.1g has come into prominence. Also, IEEE 802.11b is superior to IEEE 802.11a in terms of the communication distance.
IEEE 802.11n has been lately stipulated as a technique standard to overcome the limitation of the communication speed which has been admitted as a weak spot of the WLAN. IEEE 802.11n aims to increase the speed and reliability of a network and extend an operation distance of a wireless network. In detail, IEEE 802.11n supports high throughput (HT) of more than a maximum 540 Mbps as a data processing speed, and is based on a multiple input and multiple output (MIMO) technique using multiple antennas at both ends of a transmission unit and a reception unit to minimize a transmission error and optimize a data rate. Also, IEEE 802.11n standard can use orthogonal frequency division multiplex (OFDM) to increase the speed as well as using a coding scheme in which several duplicates are transmitted to enhance data reliability.
Meanwhile, a basic access mechanism of an IEEE 802.11 MAC (Medium Access Mechanism) is a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) combined with binary exponential backoff. The CSMA/CA mechanism is also called a DCF (Distributed Coordination Function) of IEEE 802.11 MAC, basically employing a ‘listen before talk’ access mechanism. In this type of access mechanism, a station (STA) first listens to a radio channel or a medium before starting a transmission. Upon listening, when it is detected that the medium is not is use, the listening station starts its transmission. Meanwhile, when it is detected that the medium is in use, the station enters a delay period determined by a binary exponential backoff algorithm, rather than starting its transmission.
The CSMA/CA mechanism includes virtual carrier sensing as well as physical carrier sensing in which the station (STA) directly listens to a medium). The virtual carrier sensing is to complement the limitation of the physical carrier sensing such as a hidden node problem, or the like. For the virtual carrier sensing, IEEE 802.11 MAC uses an NAC (Network Allocation Vector). The NAV is a value for the station (STA), which currently uses the medium or has authority to use the medium, to indicate a time remaining for the medium to be available, to other stations (STA). Thus, the value set as the NAV corresponds to a period during which the medium is due to be used by the station (STA) which transmits a corresponding frame.
One of procedures for setting the NAV is a procedure of exchanging an RTS (Request To Send) frame and a CTS (Clear To Send) frame. The RTS frame and the CTS frame include information informing reception stations (STA) about an upcoming frame transmission to delay a frame transmission by the reception stations. The information may be included in a duration field of each of the RTS frame and the CTS frame. When the RTS frame and the CTS frame are exchanged, a source station (STA) transmits an actual frame desired to be transmitted to a target station (STA).
In this case, however, the CSMA/CA-based channel access method does not have high efficiency. For example, when a PHY SAP (Service Access Point) provides throughput of 1 Gbps, a MAC SAP can provide throughput of merely 50% to 60% of 1 Gbps provided by the PHY SAP.
In addition, in order to effectively use a radio channel (or a wireless channel) in the VHT system, it is required to support a MU-MIMO (Multi User Multiple Input Multiple Output) to allow several VHT STAs to simultaneously use a radio channel. In the system supporting the MU-MIMO, when a channel is accessed through a CSMA/CA, several STAs must transmit and receive an RTS frame and a CTS frame to and from an AP. Here, a scheme for avoiding collision between the RTS frames and CTS frames transmitted by the several STAs is also required.
Considerations of a contention free (CF)-based channel access method, away from a contention-based channel access method, are required to overcome the foregoing problems in the VHT wireless LAN system supporting a data transmission by the MU-MIMO.